1. Field of the Invention
The present invention relates to an optical system including a movable mirror for tracking corrections which is used in a memory apparatus and the like using a magneto-optical disk, and more particularly to an optical system which is thin and compact and in which optical components are effectively arranged.
2. Description of the Related Art
A typical optical system uses a movable mirror and a drive portion (galvano-mirror) for the mirror so as to reproduce and record information with light beams. This kind of optical system is mainly used in a memory device which uses a magneto-optical disk.
FIGS. 15 and 16 show arrangement examples of a conventional optical system using a galvano-mirror.
As shown in FIG. 16, a galvano-mirror G.sub.1 drives a movable mirror 50. Numerals 50a, 51 and 52 denote a reflecting face of the movable mirror 50, a holder for supporting the movable mirror 50 and a coil wound around the peripheral surface of the right half portion of the holder 51 in the figure, respectively. The holder 51 is connected to the leading end of a support 53a through an elastic member 55, for example, a rubber. The support 53a is integrally connected to a magnetic yoke 53 which is U-shaped and inside of which a pair of magnets 54 are fixed. The magnets 54 are located opposite to upper and lower sides of the coil 52 so that opposite poles of the magnets 54 are opposite to the upper and lower sides of the coil 52, respectively. Therefore, when electric power is supplied to the coil 52, the holder 51 and the movable mirror 50 are rotated in the .alpha. direction due to currents flowing on the upper and lower sides of the coil 52 and the magnetic poles of the magnets 54. The holder 51 is made rotatable by the deformation of the shape of the elastic member 55, and the center M--M of rotation of the holder 51 corresponds to an axial line passing through the elastic member 55.
In an optical system shown in FIG. 15 , a laser beam emitted from a semiconductor laser 4 in a detection optical unit A mounted in a fixed portion is changed into a parallel beam by a collimating lens 5. The parallel beam is reflected by a reflecting face 50a of a galvano-mirror and a reflecting prism 2b and transmitted to a movable unit B. The movable unit B moves along a magneto-optical disk D and in the radial direction of the disk D. The light beam is reflected by a reflecting prism 2a in the perpendicular direction and condensed by an objective lens 1, thereby forming a micro spot S on a recording face of the disk D. The reflected beam from the disk D returns in a path through which the beam reached the disk D, is reflected by a beam splitter 7 in the perpendicular direction and detected by an unillustrated light receiving detection optical system.
In the conventional arrangement example shown in FIG. 16, the laser beam emitted from a semiconductor laser 4 is reflected upward by a reflecting prism 3 and by the reflecting face 50a of the galvano-mirror and transmitted to a reflecting mirror 2a in a movable unit B.
Referring to FIGS. 15 and 16, the reflecting face 50a is rotated on the axis M--M in the .alpha. direction due to the current applied in the coil 52. This rotation displaces the reflecting optical axis of the reflecting face 50a. Then, the micro spot S formed on the disk D is moved in the radial direction, and thus a tracking correction operation is performed.
In the above conventional optical system using a galvano-mirror, the rotation axis M--M of the movable mirror 50 extends parallel to the reflecting face 50a and in the tangential direction of the disk D. Therefore, it is necessary to transmit the light beam toward the movable unit B after reflecting the light beam upward by the reflecting face 50a, as shown in FIG. 15, or by the reflecting prism 3 as shown in FIG. 16. This makes it necessary to arrange the optical system in the detection optical unit A mounted in the fixed portion so as to have two tiers, and as a result, the optical system is made thicker. Furthermore, as shown in FIG. 16, since the drive portion of the galvano-mirror is disposed to be inclined, the optical system is made even thicker. In addition, since the incident optical axis into the reflecting face 50a and the reflecting optical axis of the same are disposed along the meridian plane (a plane along the sheet planes including FIGS. 15 and 16), it is required that the optical axis of the detection optical unit A extend in the radial direction (the right direction in FIGS. 15 and 16). Therefore, the length of a portion of the detection optical unit A projecting to the right in the figures is increased, and the diameter of the disk D and the length of the detection optical unit A are added to the width of the whole system. This prevents the system from being made thinner and more compact.
For example, an optical system shown in FIG. 17 (a top view thereof) is disclosed in Japanese Utility Model No. Sho 63-12335. This optical system performs a tracking correction operation by rotating a movable mirror 50 on an axis M perpendicular to a disk D, horizontally displacing a light beam transmitted from a detection optical unit A by a reflecting face 50a, transmitting the light beam to an objective lens 1 by a reflecting mirror 2a, and displacing the detected light in the radial direction with respect to the disk D. In this arrangement, it is not necessary to vertically transmit the reflected light as shown in FIGS. 15 and 16, and it is thus possible to make the system thinner. However, since it is required in this arrangement to extend the optical axis of the optical components, such as a wavelength plate 8 and a Wollaston prism 9, which constitute the detection optical unit A for transmitting the light beam to the movable mirror 50, in the radial direction, the width W of the system in the radial direction is large, as is the case with the widths of the systems shown in FIGS. 15 and 16. Furthermore, when a movable unit B is moved in the radial direction in the arrangement shown in FIG. 17, since it is necessary to move the movable mirror 50 in the radial direction together with the movable unit B, the construction of the system is complicated. In addition, if the movable mirror 50 and a drive portion for the movable mirror 50 are mounted in the movable unit (a pickup) B, the pickup is too heavy to follow a high speed access.